Method for cleaning a crystal microbalance

ABSTRACT

A method of removing solid particles from the active surface of piezoelectric crystal microbalances by driving the crystal into a multiplicity of vibrational modes. When used to measure mass on its surface, the crystal vibrates in the plane of the crystal surface. But, by applying a current substantially greater than used in making mass measurements and scanning across a range of frequencies spanning the natural clean resonant frequency of the crystal, a multiplicity of resonances are excited and substantially all of the solid particles are removed from the crystal surface.

United States Patent [191 King, Jr. et al.

[ Dec. 23, 1975 METHOD FOR CLEANING A CRYSTAL MICROBALANCE [75]Inventors: William H. King, Jr., Florham Park;

Gideon M. Varga, Jr., Clark, both of NJ.

[73] Assignee: Exxon Research and Engineering Company, Linden, NJ.

[22] Filed: June 5, 1974 [21] App]. No.: 476,601

3,653,253 4/1972 Olin 73/28 Primary Examiner-Barry S. Richman AssistantExaminerBradley R. Garris Attorney, Agent, or Firm-Harold N Wells; F.Donald Paris ABSTRACT A method of removing solid particles from theactive surface of piezoelectric crystal microbalances by driving thecrystal into a multiplicity of vibrational modes. When used to measuremass on its surface, the crystal vibrates in the plane of the crystalsurface. But, by applying a current substantially greater than used inmaking mass measurements and scanning across a range of frequenciesspanning the natural clean resonant frequency of the crystal, amultiplicity of resonancesare excited and substantially all of the solidparticles are removed from the crystal surface.

7-Claims, 4 Drawing Figures US. Patent Dec. 23, 1975 Fig.1.

Sheet 1 of 2 0 AFOUT o fPWIP US. Patent Dec. 23, 1975 Sheet 2 of 2OUTPUT METHOD FOR CLEANING A CRYSTAL MICROBALANCE BACKGROUND OF THEINVENTION Piezoelectric crystals have been used as microbalances formeasuring very small quantities of materials. In U.S. Pat. No.3,164,004, the use of such crystals is generally discussed, thepreferred method being to place a substrate on the crystal which canabsorb from a gas, stream the material to be measured. The absorbedmaterial increases the effective mass of the crystal, changing itsresonant frequency in proportion to the amount absorbed, thus makingpossible using the crystal as a weighing device.

The crystals have also been used for measuring solid particles, with andwithout a substrate which causes the particles to adhere to its surface.Disclosure of such applications is given in U.S. Pat. Nos. 3,653,253;3,561,253; and 3,715,911. When measuring material absorbed from the gasphase, it is possible to desorb the material from the substrate, leavingit clean and ready to receive a new sample. However, measurement ofsolid particles presents a different cleanup problem. The crystals areinsensitive to particles lying loosely on the crystal surface. Thevibration is in the direction of the plane of the crystal in the typicalAT-cut crystals, which are commonly used for this application. Thus, thesolid particles must be securely attached to the crystal surface if theyare to increase the crystal mass and so be measured. An adhesive issometimes used to attach the particles or electrostatic forces may bedeveloped to accomplish the same result without an adhesive. Once thesolid particles have adhered, however, it is necessary to remove them ifanother measurement is to be made with the same crystal. This may bedone in a number of ways, as is noted in U.S. Pat. No. 3,653,253 (column6, lines 4 through 13).

A similar problem arises when the crystal is to be used with anabsorptive substrate for measurement of moisture in air. Measurementsmade in a very dusty atmosphere could be hindered by dust attached tothe crystal. The crystal could measure the weight of the dust itselfinstead of the moisture absorbed on the substrate. Also, moistureabsorbed by the dust can be added to that absorbed by the substrate.

Assuring a clean crystal in dusty atmospheres or removing solidparticles attached to crystals represents a significant problem,especially in situations where the instrument is to be operatedautomatically without the attention of an operator over significantperiods of time. The present invention has overcome these difficultiesand makes possible essentially complete cleaning of piezoelectriccrystals which have become intentionally or inadvertently affected bythe presence of dust particles.

SUMMARY OF THE INVENTION While a normal piezoelectric crystal (AT-cut)oscillates parallel to the plane of its surface when using a crystal asa microbalance, it has been found possible to remove dust particlesattached to the surface by forcing such crystals into many differentvibrational modes, that is, contrary to the normal use as amicrobalance. With sufficient amplitude, such vibrations are sufficientto remove solid particles from the crystal surface without additionalassistance. In order to fully clean it, a crystal is forced to vibrateat a series of frequencies spanning its natural resonant frequency andat a relatively high power compared to that at which the crystalvibrates during its measuring mode. When driven at relatively highpower, the crystal can be made to oscillate in many vibrational modes.By scanning a range of frequencies, beginning below and ranging to abovethe principal natural resonant frequency, it has been found possible toexcite many of the crystals minor resonances, the net effect of which isto clear all of the dust particles from the surface without any furtherphysical removal by blowing, sweeping, or other such prior art means. Inthe preferred embodiment, the scanning process is repeated in aplurality of cleaning steps until the crystal is fully restored to itsoriginal clean condition.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a typicaltransistorized oscillator circuit used when the crystal is operated as amicrobalance.

FIG. 2 illustrates a crystal impedance meter circuit which may be usedto force the crystal to oscillate in its cleaning mode.

FIG. 3 illustrates schematically an embodiment of the invention whereina single crystal is transferred between a frequency measuring oscillatorcircuit and the de-dusting circuit.

FIG. 4 shows an alternative embodiment in which the crystal is switchedfrom one circuit to another.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A piezoelectrical crystal usedas a microbalance will normally be operated in an oscillator circuitdesigned for this purpose. A typical transistorized Pierce oscillatorcircuit is illustrated in FIG. 1. Other similar oscillators may be usedto make such measurements, but the details of such circuits are notconsidered to be part of the present invention. The function of any suchoscillator circuit is generally to apply a voltage to the piezoelectriccrystal which causes it to oscillate and thereafter feeds back powerinto the crystal to maintain the vibration at its natural resonantfrequency. This resonant frequency is measured and referred to that of aclean crystal, the difference in frequency being proportional the massof the material deposited or absorbed on the crystal surface.

It must be appreciated that in this normal operation the amount of powerwhich is applied to the crystal is relatively low and that the crystalis usually cut (AT- cut) so that it vibrates parallel to the plane ofits surface. In such a vibration, particles attached to the crystalsurface will have a significant effect on the crystals resonantfrequency. However, if the crystal moves independently of particleswhich are loosely resting on its surface it will fail to indicate theirpresence. When piezoelectric crystals are used to detect solid particlesin atmospheric pollution testing this problem has been solved in twoways. First, adhesive materials are applied to the crystal surface towhich the solid particles can be finnly attached; and second,electrostatic charges are generated on the dust particles opposite to 3that of the crystal surface. These adhering particles must be removed ifthe crystal is to be used in repetitive measurements. Various methodsmay be used as suggested in US. Pat. No. 3,653,253, for example, blowingoff the solid particles or washing with a cleaning fluid. Anotherpossibility would be to brush away the solid particles. It is alsosuggested in that same reference that by driving the crystal at a veryhigh current the particles may be dislodged from the surface and thenmay be blown away by a jet of clean air. It has been found, however,that such semi-manual cleaning is not necessarily required, that byproper imposition of preselected frequecies on the crystal, spanning therange of its principal natural resonant frequency, the essentiallycomplete cleaning can be achieved.

Removal of solid particles from the crystal surface may also be aproblem where the crystal is not being used for measuring solidparticles but for measuring the absorption of moisture on a specialsubstrate applied to the crystal. In such a situation, the solidparticles will interfere with the normal function of the crystal andthey must be removed if the crystal is to properly measure the absorbedgas. As has been mentioned, if the dust is tightly bound to the crystal,it will substantially affect its resonant frequency, but if looselyattached to the crystal, the dust may have little or no effect. However,where substantial amounts of dust are present, even without significantattachment of the dust, a mechanical damping effect may cause a shift inthe resonant frequency, which could lead to erroneous results.Particularly where the crystal microbalance is to be used unattended inautomatic routine operation, it is essential to assure that the crystalis free of any such interfering solid particles. Accordingly, thepresent invention provides a means by which the crystal may be cleanprior to making the measurement to assure that any such interference ofsolid particles has a negligible effect upon the result. It has beenfound that if the dust loading is high enough, that it is possible to socompletely damp the crystal oscillation that the normal Pierceoscillator circuit of FIG. 1 is unable to force the crystal intoresonant vibration. According to the present invention, however, thecrystal may be forced into vibration at other its principal resonantfrequency and at relatively high power in order to excite resonancesnormal to the crystal surface, loosening and effectively removing solidparticles.

It has been recognized in the prior art that the surface of a crystalwill exhibit various patterns of vibration, depending upon the frequencyat which it is excited. In fact, one mode of discovering the patterns ofvibration has been to apply a fine dust to the surface of the crystaland then to observe the rearrangement of the dust on the crystal whichdepends upon the mode of vibration. When interacting with the normaloscillator circuit, an AT-cut piezoelectric crystal vibrates parallel tothe surface of the crystal. It has been discovered, however, that byapplying a significant amount of power to the crystal at other than itsnormal resonant frequency, that many minor resonances can be excited. Itis this abnormal mode of operation which causes solid particles to bedislodged. If one applied power at only a single frequency, the dustparticles would tend to try to arrange themselves so as to collect onthe nodal points and to leave the portions of the crystal which arevibrating with a maximum amplitude in much the same way as experimentaldeterminations of the mode of crystal vibration were made. However, ithas been found that by beginning at a frequency significantly below thenatural resonant frequency of the crystal, scanning over a range offrequencies extending through and beyond the natural resonant frequencyand applying sufficient power, it is possible to excite a large numberof resonances so that during the scanning procedure all of the crystalsurface is subjected to a substantial vibration, not only dislodging theparticles, but removing them entirely. This is true even when thecrystal is in a horizontal position where it might be expected that thedust particles would simply be dislodged and then fall back onto thecrystal again.

In one embodiment of the invention, a crystal impedance meter similar tothat shown schematically in FIG. 2, was used to carry out the method ofthe invention. In a typical AT-cut crystal used as a microbalance andhaving a fundamental resonance of about 9 megacycles, a range offrequencies ranging from about 4 kilocycles below the resonant frequencyto 6 kilocycles above the resonant frequency was scanned at a rate ofabout 1 kilocycle per second. A low voltage was applied to the crystal,selected to avoid excessive currents when resonant frequencies werereached and thus avoiding damage to the crystal. The power appliedduring the frequency scanning varied depending upon the resistance ofthe crystal, which was continually changing as dust was being removed.The power dissipated also depended upon whether a natural resonance wasbeing excited. It was found that maximum power had to be limited inorder to avoid physical damage to the crystals and, accordingly, thepower was generally limited with a 9 megacycle crystal to about one-halfwatt maximum input. This proved to be sufiicient power input tocompletely remove the dust when the scanning of the frequency range wasrepeated 5 to 10 times and required no other assistance. A test of thecrystals ability to measure absorbed moisture in its clean condition andafter having been loaded with dust and then cleaned according to theinvention indicated that the response of the crystal was substantiallythe same after cleaning as before. Thus, the cleaning method of theinvention was effective in removing dust and permitted repeatablemeasurements, which is an essential feature of such a cleaningtechnique.

FIG. 3 illustrates schematically one mode of applying the invention. Thecrystals 10 are typically available in a plug-in format and can beinserted into the Pierce oscillator circuit 12 or other oscillatorcircuit for making measurements of mass deposited on the surface,acquired through absorption of a gas on a substrate or by deposition ofsolid particles. After the measurements have been made and cleaning isnecessary, it would be possible to unplug the crystal and to place itinto a de-dusting circuit 14 which would provide the scanning offrequencies at relatively high power compared to the Pierce oscillatorcircuit 12, which simply seeks out and maintains the natural vibratingfrequency. The crystal impedance meter illustrated in FIG. 2 could beused as the de-dusting circuit into which the dirty crystal is plugged.

In remote use where self-cleaning feature of the crystal would beimportant, the arrangement of FIG. 4 would be preferred, wherein thecrystal 10 is simply switched from the measuring circuit 12 to thecleaning circuit 14, depending upon the mode in which the crystal is tobe operated. In FIG. 4 the crystal 10 is viewed from that shown in FIG.3, illustrating that a typical crystal 10 is a flat circular disc ontowhich electrodes a and b have been secured, one on either side of thecrystal. It will be appreciated that a dust particle lying on thesurface of such a crystal 10 might have little effect when the crystalis vibrating parallel to the plane of the crystal, but such a particlesecurely attached to the crystal could vibrate with it, afiecting itsmass and its resonant frequency. In the present invention the crystal isforced to vibrate in many vibrational modes, not just in the plane ofthe crystal, so that any particle which rests or is secured to itssurface may be dislodged by the vibrations. Such a use of the crystal iscompletely contrary, not only to its normal use in measuring mass, butto the general conception of the operation of such crystals, which arecarefully cut in order to obtain vibration parallel to the surface.Since it is necessary when dislodging solid particles to force thecrystal into a mode of operation which substantially departs from thenormal method of operating such crystals, the present inventionrepresents a major departure from teachings of the prior art.

The invention has application in situations where crystals are used formeasuring of solid particles and must be cleaned prior to a subsequentmeasurement, but it also may be applied where the interference of dustparticles must be avoided. Various electrical circuits may be used inorder to provide for both measurement or cleaning according to theinvention, without departing from the essence of it. The foregoingdescription of the preferred embodiments is for illustration of themethod only and should not limit the scope of the invention which isdefined by the claims which follow.

What is claimed is:

l. A method for removing particles from the surface of a piezoelectriccrystal used in its measuring mode as a rnicrobalance, comprising thesteps of:

a. applying a variable electrical AC voltage to said crystal, therebygenerating variable frequency signals in a frequency range extendingfrom a lower frequency below the natural clean resonant fre- 6 quency ofsaid crystal to an upper frequency above said natural clean resonantfrequency of said crystal, whereby said crystal vibrates in differentvibrational modes corresponding to said frequency range spanning saidnatural clean resonant frequency; and

b. vibrating said crystal as in step (a) at a relatively high powercompared to the power at which said crystal vibrates in its measuringmode, whereby said particles are dislodged from and vibrated off of saidcrystal and thereby said crystal is substantially cleaned of anyparticles present on the surface thereof.

2. The method of claim 1 including the step of repeating the process ofvibrating said crystal in substantially all of its vibrational modesuntil said crystal is fully restored to its original clean condition,whereby said crystal vibrates in a plurality of directions causingremoval of said particles from the surface thereof and wherein theexcitation of said crystal is in a direction normal to said crystalsurface.

3. The method of claim 2 wherein steps (a) and (b) are repeated from5-10 times.

4. The method of claim 1 wherein said frequency range is from 4 kc belowsaid natural clean resonant frequency to 6 kc above said natural cleanresonant frequency.

5. The method of claim 1 including the step of limiting the power inputto said crystal to about 0.5 watts and wherein said crystal has anatural clean resonant frequency of 9 megacycles.

6. The method of claim 1 including the step of scanning said frequencyrange at a rate of l kc/second.

7. The method of claim 1 including the step of transferring said crystalfrom its modes required for removal of said particles from said surfaceto its measuring mode.

1. A METHOD FOR REMOVING PARTICLES FROM THE SURFACE OF A PIEZOELECTRICCRYSTAL USED IN ITS MEASURING MODE AS A MICROBALANCE, COMPRISING THESTEPS OF: A. APPLYING A VARIABLE ELECTRICAL AC VOLTAGE TO SAID CRYSTAL,THEREBY GENETATING VARIABLE FREQUENCY SIGNALS IN A FREQUENCY RANGEEXTENDING FROM A LOWER FREQUENCY OF SAID CRYSTALS THE NATURAL CLEANRESONANT FREQUENCY OF SAID CRYSTAL TO AN UPPER FREQUENCY ABOVE SAIDNATURAL CLEAN RESONANT FREQUENCY OF SAID CRYSTALS, WHEREBY SAID CRYSTALVIBRATES IN DIFFERENT VIBRATIONAL MODES CORRESPONDING TO SAID FREQUENCYRANGE SPANNING SAID NATURAL CLEAN RESONANT FREQUENCY; AND B. VIBRATINGSAID CRYSTALS AS IN STEP (A) AT A RELATIVELY HIGH POWER COMPARED TO THEPOWER AT WHICH SAID CRYSTAL VIBRATES IN ITS MEASURING MODE, WHEREBY SAIDPARTICLES ARE DISLODGED FROM AND VIBRATED OFF OF SAID CRYSTAL ANDTHEREBY SAID CRYSTAL IS SUBSTANTIALLY CLEANED OF ANY PARTICLES PRESENTON THE SURFACE THEREOF.
 2. The method of claim 1 including the step ofrepeating the process of vibrating said crystal in substantially all ofits vibrational modes until said crystal is fully restored to itsoriginal clean condition, whereby said crystal vibrates in a pluralityof directions causing removal of said particles from the surface thereofand wherein the excitation of said crystal is in a direction normal tosaid crystal surface.
 3. The method of claim 2 wherein steps (a) and (b)are repeated from 5-10 times.
 4. The method of claim 1 wherein saidfrequency range is from 4 kc below said natural clean resonant frequencyto 6 kc above said natural clean resonant frequency.
 5. The method ofclaim 1 including the step of limiting the power input to said crystalto about 0.5 watts and wherein said crystal has a natural clean resonantfrequency of 9 megacycles.
 6. The method of claim 1 including the stepof scanning said frequency range at a rate of 1 kc/second.
 7. The methodof claim 1 including the step of transferring said crystal from itsmodes required for removal of said particles from said surface to itsmeasuring mode.